A key element of the Epiphyte Direct Air Capture (DAC) system is the sorbent heater, since the capture cycle uses heat to release (desorb) the captured CO2 and regenerate the sorbent:
Before testing the heating system in the actual Epiphyte, I built a prototype version to check out the design concept and understand its behavior.
Heating System Requirements
The main requirement is for the system to be safe from electrical hazards and fire risks. It needs to heat the sorbent to the required temperature, without getting so hot that the sorbent is damaged. It needs to be able to transfer the heat to the sorbent by direct conduction, since it may need to operate in a vacuum.
Design Choices
In the interests of size, cost, and flexibility, I selected resistance wire (Nichrome) as the heating element. Since in the Epiphyte system this wire will be in physical contact with the metal mesh containing the sorbent, it needs to be electrically insulated. Only bare Nichrome wire is readily available, so I cover the bare wire with a braided fiberglass sheathing; this material is able to withstand high temperatures while being a relatively good thermal conductor.
I chose an 18AWG nichrome wire from Remington, with a resistivity of 0.4 ohms/foot.
For safety reasons, I power the system from a 24-VDC supply; any line voltage (110VAC) connections are confined to an electrical box. A pulse-width modulation (PWM) scheme is used, in which the DC supply is switched on and off at about 1kHz, and the exact amount of power delivered to the heating wires is controlled by the processor by varying the duty cycle (width) of the pulses. The basic electrical schematic is shown here (drawn using LTspice):
The processor controls the heat by applying the PWM switching signal from a GPIO output (represented by source V2) to the base of the NPN transistor; the collector drives the gate of a high-power P-channel MOSFET, which alternately connects and disconnects the 24V power (V1) and the heater wire R1; and R2 is a current sensing resistor (not used yet).
The heating wire used in Epiphyte will be about 6 feet long, arranged in a zig-zag pattern across each face of the sorbent, as pictured here:
With the above information, we can calculate the maximum power available from the heating wires as
P = V^2 / R = (24V)^2 / (0.4 ohms/foot * 6 foot) = 240W
which will be reduced in practice proportionally to the duty cycle applied.
Construction of the Heater Prototype
To replicate the actual arrangement that will be used in Epiphyte, I built a rough prototype of the heater on a piece of plywood, using the same amount of wire, and not in contact with the wood. Note the two themocouples that will be used in testing.
The box containing two standard 24VDC, 12A supplies is shown here before installing the lid:
The complete setup, including the processor and the electronics described above, is shown here:
An oscilloscope view of the waveforms is shown. The top trace shows the control signal from the processor, at 2V/division; this is a standard 3.3-V GPIO output. The bottom trace is the voltage across the heating wire, at 5V/div.
Measurements
The measurements consist of operating the heating system with a specified duty cycle (10%, 20%, 30%, and 40%) and logging the temperature as it rises from ambient. Thermocouples are placed at two points: in contact with the bare wire (but insulated electrically with Kapton tape); and on the outside of the sheathing. The chart shows the results of the four cases, where we can see the temperature ramping up until it levels off at a maximum value; for each case, the upper trace represents the bare wire measurement, the lower trace is the temperature on the outside of the sheathing. Note: I didn’t go beyond 40% because I didn’t want to start a fire!
Discussion
The shape of the curves looks well-behaved and should be easy to model theoretically for designing the temperature control algorithm. Clearly the specific results will be different in the actual system owing to the presence of the sorbent, frame, and metal elements, so the same measurement will need to be repeated after installation. However, the current data is useful for suggesting ideas for the algorithm, which will probably be a software PID control system with continuous temperature measurements for feedback.
The temperature drop through the fiberglass sheathing is an unfortunate inefficiency, and we need to reconsider the use of it in favor of a better heat-conductive high-temperature insulator.